Contactless web support guides are incorporated in printing machines, usually when it is required to print both sides of a web of material without intermediate drying between the two printing processes. In effect, they are air bearing devices which replace specific rollers of the printing machine, thereby preventing smudging. Typically, such support guides are used to change the direction of motion of a web (usually through 90.degree.), in which case they are referred to as "air turns". Air turns are usually used in pairs to reverse the direction of motion of a web. Similar support guides (known as "air bars") are used to support and guide webs without changing their direction of motion.
There are two main problems which a successful contactless support guide must overcome. Firstly, the air pressure between the underside of the web and the support face must be even, and as low as possible consistent with the keeping of a layer of air between the two surfaces, even though the web tension varies and the escape of air at the web entry and exit regions of the support face causes a reduction in pressure at these regions. Any unevenness of air pressure will tend to cause ballooning out of the web, which leads to instability and possibly bursting. Secondly, it is essential that the contactless support guide maintains lateral stability of the web, whilst preventing the web from touching the support face. The constant demand for ever increasing outputs, with the resulting increased web speeds, has led to longer unsupported web runs in the latest machines, particularly in their drying units. Known contactless support guides are inadequate in this respect. They are also extremely noisy.
The aim of the invention is to provide a contactless support guide which maintains the stability of the web at high speeds, and is quieter than known devices.